1. Field of the Invention
The present invention relates to a thin-film type thermal printing head for use in a printing portion of facsimiles, printers or the like, and a method of manufacturing the same. More particularly, the present invention pertains to the structure of wiring connected to heating resistors, the structure of solder connecting portions which provide an electrical connection to an external circuit by means of soldering, and a method of manufacturing such structures.
2. Description of Prior Art
The structure of a conventional thermal printing head will be described below with reference to FIGS. 1 to 3.
FIG. 1 is a perspective view of the conventional thermal printing head. Driving ICs 110 and one end of a flexible printed board 120 are soldered to a thermal printing head base 100 bonded to a heat sink 80. Connectors 130 are soldered to the other end of the flexible printed board 120. Signals for driving the head enter the thermal printing head through the connectors 130 from an external circuit, and control the driving ICs 110 and thereby drive heating resistors (not shown).
FIG. 2 is a plan view of the essential parts of the conventional thermal printing head base 100. Heating resistors 20 formed on a high-resistance substrate 10 are electrically connected to a common wiring 50 and individual wirings 30. The heating resistors 20 are also connected to an external circuit (not shown) through the flexible printed board 120 at solder connecting portions 61 connected to electrodes of the driving ICs 110 and at solder connecting portions 62 connected to electrode terminals of the flexible printed board 120.
FIG. 3 is a sectional view taken along the line A--A' of FIG. 2. A heating resistor layer 21 made of an alloy of chromium and silicon is formed on the high-resistance substrate 10 composed of a ceramic layer 11, a glaze layer 12 and tantalum pentaoxide layer 13 by sputtering, and a 0.1 .mu.m thick chromium layer 31 and a 0.8 .mu.m thick aluminum layer 34 are then formed on the heating resistor layer 21 in sequence by sputtering to form the wiring 30. Thereafter, an unnecessary portion of the wiring 30 and that of the heating resistor layer 21 are removed by the photolithographic process to form the heating resistors 20.
Next, to protect the heating resistors 20 and the wiring 30, a protective layer 40 consisting of two layers is formed first by forming a silicon dioxide layer 41 to a thickness of 4.0 .mu.m by sputtering and then forming through-holes by the photolithographic process and then by forming a polyimide layer 42 to a thickness of 3.5 .mu.m and then forming through-holes by the photolithographic process. Subsequently, the common wiring 50 and the solder connecting portions 60, each composed of a chromium layer 51, a copper layer 52 and a gold layer 53, are formed at the same time using both the sputtering and the photolithographic process. Thereafter, an abrasion resistant protective layer 71 made of, for example, silicon nitride, is formed selectively on both the common wiring 50 and the heating resistor 20 by the plasma CVD process.
The thermal printing head of the above-described type may be employed in the thermal printing method. In that case, a thermal printing paper is moved, perpendicularly to the paper on which FIG. 3 is depicted, from the right to the left by a platen roller (not shown) while being pressed against the heating resistors. In consequence, lees 91 of the printing paper remain at the shoulder of the common wiring 50, deteriorating contact of the printing paper with the heat transmitting portion of the upper portions of the heating resistors. This necessitates cleaning of the head once a month in a case where the head is used at a normal frequency.
In the above-described conventional thermal printing head, the protective layer 40 is made up of the silicon dioxide layer 41 and the polyimide layer 42 to attain reliability because the easily corrosive aluminum layer 34 is used to form the wiring 30. The thickness of the silicon dioxide layer 41 is particularly important. That is, to prevent corrosion of the aluminum layer 34, the silicon dioxide layer 41 must have a thickness of 4.0 .mu.m or above. The silicon dioxide layer 41 is formed on the heating resistor 20 also, and the thickness thereof thus greatly affects the printing characteristics. In the case where aluminum is used as a metal for wiring, a level of printing energy must therefore be enhanced because of the thickness of the silicon dioxide layer 41. Further, the polyimide layer 42 is used as a stress relieving film to prevent the glaze layer 12 from being cracked by the stress applied thereto from the electrode connecting solder when the driving ICs are mounted.
The use of the wiring made up of at least two layers, as in the case of the above-described conventional thermal printing head, e.g., the use of the wiring made up of, for example, a lower chromium layer and an upper aluminum layer, as disclosed in Japanese Patent Unexamined Publication No. 61-43449, assures economic wiring substrate. However, this necessitates formation of another solder connecting metal layer on the aluminum layer because the normally employed solder does not alloy with aluminum.
In the solder connecting portion 60 employed in the above conventional thermal printing head, the copper layer 52 is connected to a solder, the gold layer 53 has a function of preventing oxidation of the surface of the copper layer 52, and the chromium layer 51 has a function of bonding the solder connecting portion 60 to a layer disposed below it.
Japanese Patent Unexamined Publication No. 63-28665 discloses a thermal printing head which employs copper as a wiring metal and an alloy of nickel and copper as a solder connecting metal. Although the alloy of nickel and copper ensures excellent solder connection, the number of metal layers in the thermal printing head is increased, making the manufacturing process complicated. Furthermore, no consideration is given to a change in the thickness of the protective layer caused by a change in the wiring metal.
Thus, in the conventional thermal printing heads, the lees 91 of the printing paper easily remain at the shoulder of the common wiring. This makes frequent cleaning of the head necessary. Furthermore, in a case where aluminum is used as a wiring metal, the thickness of the protective layer must be increased. This prevents reduction in the power consumption of the thermal printing head. Also, in a case where aluminum is used as a wiring metal, since the electrical connection with an external circuit is achieved by the soldering process, a solder connecting metal other than that used in the wiring must be used.
As stated above, the conventional thermal printing heads have disadvantages in that the thickness of the protective layer must be increased and the level of printing energy must therefore be enhanced because of the use of aluminum as the wiring metal, in that the use of different metals for the wiring and for the solder connecting portions and common wiring makes the overall configuration complicated, and in that frequent cleaning is required, making the operation of the head uneconomical.